This invention relates generally to ink jet arrays which include a plurality of ink jet channels with each channel having a chamber, an inlet to the chamber, an orifice from the chamber, and transducer means coupled to the chamber for ejecting droplets of ink from the chamber as a function of the state of energization of the transducer. More particularly, this invention relates to an efficient method of fabricating a high-density ink jet array.
Within the art of ink jet printing, it is often desirable to employ a print head configuration which permits the utilization of a plurality of ink jets in a densely packed array such that a reasonably large area of a copy medium may be printed simultaneously. This is especially the case in the printing of alphanumeric information in which the resolution of individual characters as well as the speed of printing are of utmost importance.
One suitable such printer is described in U.S. Pat. No. 4,459,601, issued July 10, 1984 to Stuart D. Howkins, assigned to the assignee of the present invention and incorporated herein by reference. In that arrangement, an ink jet apparatus of the demand or impulse type comprises a chamber and an orifice in which droplets of ink are ejected in response to the state of energization of a transducer which communicates with the chamber through a foot forming a movable wall. The transducer expands and contracts, in a direction having at least one component extending parallel with the direction of droplet ejection through the orifice, and is elongated in such direction, the electric field resulting from the energizing or drive voltage being applied transversely to the axis of elongation.
One problem common to all high-speed, high-resolution, drop-on-demand ink jet printers occurs because the jets of an array are spaced very close to one another. That is, the response of one jet in an array to its drive voltage can be affected by the simultaneously application of a drive voltage to another nearby jet. This can result in a phenomenon, known in the art as "mechanical cross-talk", where pressure waves are transmitted through the solid material in which the jets are formed, or in another phenomenon known in the art as "electrical cross-talk", where relatively large drive voltages necessary for substantial displacement of transducers utilized in the prior art cause the subsequent pulsing of an inappropriate jet.
While the risk of electrical cross-talk between ink jets in an array utilizing the teachings of U.S. Pat. No. 4,459,601 as discussed above will be minimized, the risk of mechanical cross-talk remains. One approach which alleviates this problem, however, is discussed in U.S. Pat. No. 4,439,780, issued Mar. 27, 1984 to Thomas W. DeYoung and Viacheslav B. Maltsev, assigned to the assignee of the present invention and incorporated herein by reference. In that arrangement, an ink jet array comprises a plurality of elongated transducers coupled to a plurality of ink jet chambers, the transducers being supported only at their longitudinal extremities. The support at the extremity remote from the chamber is provided such that no longitudinal motion along the axis of elongation of the transducers occurs, while the support at the other extremity includes bearings which substantially preclude lateral movement of the transducers transverse to the axis of elongation but permit the longitudinal movement thereof along the axis, thus minimizing mechanical cross-talk between jets within the array.
As is more fully disclosed in the aforedescribed U.S. Pat. No. 4,439,780, the coupling means may comprise a foot attached to the transducer and the bearing means may comprise a hole receiving the foot. Preferably, the foot is cylindrical in cross-section and the hole is also cylindrical in cross-section with the hole slightly larger relative to the foot so as to assure no more than a line contact therebetween. The foot is subsequently "potted" within the hole by a viscoelastic material.
Other characteristic problems which are encountered in the implementation of high-speed, high-resolution impulse ink jet printers do not impact so much upon their operation, but indeed impact on their fabrication. For example, the relatively small sizes of transducer elements used in densely packed arrays make them difficult to handle. A unitary transducer array is, therefore, preferred.
One early approach towards the resolution of the above-described problem is disclosed in U.S. Pat. No. 4,072,959, which issued to Rune Elmqvist. As discussed therein, a recorder operating with drops of liquid includes a comb-shaped piezo electric transducer arranged such that individual teeth of the comb are associated respectively to a densely-packed array of ink jet chambers. Each of the transducers is immersed in a common reservoir such that energization of one transducer associated with one chamber may produce cross-talk with respect to an adjacent chamber or chambers. In other words, there is no fluidic isolation from chamber to chamber between the various transducers, or more accurately, segments of the common transducer. In addition to such cross-talk problems, the construction shown in the Elmqvist patent poses a requirement for a non-conductive ink.
It has been generally observed, however, that transducer-driven ink jets of apparently identical construction do not all operate over a single operating voltage range. This variation in operating voltage may result from such factors as variations in transducer material from piece to piece, variations in the acoustical coupling between the transducer and the remainder of the jet, or from other variations in structure which are not simple to control on a dimensional basis. These variations are troublesome in a manufacturing environment because they require the transducer driving electronics to be tuned to the jets on an individual basis where variations in transducer performance alone can be on the order of 15% to 20%. As a result, electronic configurations such a "resistor packs" which are well known in the art and used to individually tune the jets are desirably eliminated in an ink jet apparatus.
One means of reducing the variations in transducer from piece to piece is disclosed in copending application Ser. No. 902,473, filed Aug. 29, 1986, which is assigned to the assignee of the present invention and incorporated herein by reference. In that arrangement, a transducer array is produced in unitized fashion for ready assembly within an ink jet apparatus. The ink jet apparatus, in the preferred embodiment of that invention, includes a plurality of variable volume chambers, each of which is coupled to a respective element of the transducer array for ejection of ink through an associated orifice or jet. In order to fabricate the array, a monolithic slab of piezoelectric material, for example, lead zirconate titanate (PZT) is laminated to a rigid substrate such as glass by a selected thermoplastic cement. This lamina is then sized according to the desired number and dimensions and individual transducer elements and element spacing, and is subsequently diced to produce those elements. After such sizing and dicing, typically accomplished by a "dicing saw", the lamina is positioned PZT-side down with one end thereof being bonded by a structural-type electrically conductive epoxy to a shelf formed in the printer head. The other end is then operatively coupled to the variable volume chambers, and the resulting assembly is placed within an oven to cure. Once the structural-type electrically conductive epoxy has cured the oven's temperature is elevated to a point at which the thermoplastic cement will readily flow, thus facilitating the removal of the rigid substrate. Thereafter, the transducer elements can be electrically coupled by conventional means.
While the above described copending application Ser. No. 902,473, filed Aug. 29, 1986, reduces the variations in transducer material from piece to piece by producing a transducer array from a monolithic slab of piezoelectric material, variations in the acoustical coupling between the transducer and the remainder of the jet are still possible. One means of alleviating this problem, as disclosed in the aforedescribed U.S. Pat. No. 4,439,780, includes the use of a coupling means comprising a foot attached to the transducer and bearing means comprising a hole which receives the foot. Preferably, the foot is cylindrical in cross-section and the hole is also cylindrical in cross-section with the hole slightly larger relative to the foot so as to assure no more than a line contact therebetween. The coupling means further comprises a diaphragm between the chamber and the foot with a viscoelastic material sandwiched between the foot and the diaphragm assisting in the maintenance of the lateral position of the transducer at the diaphragm.
It will be appreciated, however, that an individual foot may be secured within the hole simply by means of a viscoelastic material such as silicone which is marketed under the name RTV. The ends of the transducers may be cemented to the feet by means of suitable adhesive such as an epoxy and the diaphragm may be eliminated. This "potted foot" configuration is presently preferred over the diaphragm designs illustrated herein for reasons of reliability and durability. Effective coupling of an individual transducer within a multichannel array is, nevertheless, hampered because of variations in structure which are not simple to control on a dimensional basis. A more efficient means of coupling is therefore desirable.